The increase in communication volume and setting functions lead to increasing loads such as manual installation of a new transponder (hereinafter appropriately abbreviated as “TRPN”) and maintenance of the existing TRPN. While setting of parameters such as a multi-value level and error correction is generally controlled by Software-Defined Networking (SDN), achieving a multivendor environment makes it difficult for the SDN to support all unique features of each company. Therefore, there has been desired a method wherein opposing TRPNs perform automatic control by directly exchanging supervisory (hereinafter abbreviated as “SV”) signals without using the SDN in such a situation.
To realize an automatic control function, a configuration capable of transmitting and receiving SV signals with minimum setting is useful. As one example of such a method, an Frequency Shift Keying (FSK) method for superimposing a control channel on a carrier frequency has been proposed. A method for superimposing an SV signal on an FSK signal and transmitting the superimposed signal is called an “FSK-SV method”. The FSK method is a method for changing the frequency of a carrier according to a logical value of data to be transmitted. In the case of polarization multiplexing, the receiving side receives an X-polarized wave and a Y-polarized wave without separating the both after the same value is superimposed thereon. A data value may be restored by the receiving side detecting the direction of a change in frequency.
A monitor configuration has also been proposed in which the receiving side monitors a signal transmitted by the FSK-SV method. This method is combined with control using the FSK-SV method to enable the SV signal to be transmitted and received even when no main signal is communicated.
In communication using the FSK-SV method, characteristics of an optical modulator or characteristics of an automatic bias control (ABC) circuit may cause a phenomenon that a phase rotation direction of an optical signal outputted from an IQ modulator is reversed or the phase of an optical output signal is π-radian shifted with respect to a drive signal. These phenomena are collectively called “reversal of phase rotation direction”. When the phase rotation direction is reversed, bits are reversed or values are cancelled out, and thus the FSK-SV signal may no longer be properly received.
As illustrated in FIG. 1A, in the case of a proper phase rotation state (that is, the phase rotation direction is not reversed) at the output of the IQ modulator, the drive signal rotates the phase in the same direction (for example, clockwise) and shifts the frequencies in the same direction for both of the X-polarized wave and Y-polarized wave. The X-polarized wave and the Y-polarized wave are multiplexed and transmitted to an optical transmission line, and the transmitted data value may be determined by detecting the direction of a frequency shift at the receiving side.
FIG. 1B illustrates the case where the phase rotation direction is reversed for the X-polarized wave. For the Y-polarized wave, the phase rotates clockwise and the frequency shifts in a direction indicating a logical value of inputted data. For the X-polarized wave, the phase rotates counterclockwise and the frequency shifts in the opposite direction. When the X-polarized wave and the Y-polarized wave are multiplexed, a change in frequency is cancelled out and no data may be received by the receiving side.
To solve such a problem, there has been proposed a configuration in which reversal of an FSK signal superimposed on an optically modulated signal is detected and a bias voltage of an optical modulator is controlled in the event of the reversal, thereby changing a phase difference between an I-arm modulator and a Q-arm modulator by π. Prior art documents include Japanese Patent Nos. 5278001, 5712582, and 5870728, Japanese Laid-open Patent Publication No. 2016-34078, and Z. Tao et al., “Simple, Robust, and Wide-Range Frequency Offset Monitor for Automatic Frequency Control in Digital Coherent Receivers,” ECOC 2007, pp. 1-2.